Contactless sensor measures speed and direction

Lampertheim, Germany--Because contactless sensors are essential for velocity-control applications requiring long sensor life, design engineers traditionally used inductive or capacitive sensors to measure the speed of a rotating shaft. But now a new sensor technology, based on the magnetoresistive effect, offers several advantages over these established methods.

Magnetoresistive (MR) sensors from Rechner Electronic Industries can produce signals of 20 kHz or more, while inductive or capacitive sensors cannot exceed 1 to 5 kHz. In addition, MR sensors have a preferred direction that system designers can use to distinguish the direction of shaft rotation. One MR sensor can thus provide both speed and directional information, where designers would need two inductive or capacitive sensors and additional electronics to achieve the same effect.

Other Applications* Construction equipment * Dredging
machines

An MR sensor consists of four resistances made of Permalloy® material mounted
in front of a magnet and inserted in one end of a stainless-steel tube. The
resistors are wired in a Wheatstone bridge. If a ferromagnetic or magnetized
object passes close to the active end of the sensor, it creates changes in the
magnetic fields passing through each resistance. Due to the MR effect, these
field variations cause small changes in the resistance of each element. The
changes unbalance the Wheatstone bridge and produce a differential voltage.

Rechner offers two ranges of MR sensors: The Series 300, which measures speed of rotation only, and the Series 350 which measures speed and direction. To measure the speed of a rotating shaft, a gear wheel made of ferromagnetic material mounts on the shaft; the sensor is located 1.5 to 3.0 mm from the wheel. If the wheel is made of magnetized materials, sensing distance can be 10 to 20 times greater. Output comprises a square-wave signal that drives a standard PNP transistor output stage rated at 250 mA, and an LED indicator, which flashes at the switching frequency.

To measure direction of rotation, the sensor compares the two output signals from the Wheatstone bridge. Because the MR resistances possess a preferred direction, the relative strengths of the output signals will change according to the direction of rotation of the shaft. A differential amplifier detects this change. It produces a second sensor output that is high or low depending on the direction of rotation. A two-color LED incorporated in the circuit changes color according to the direction of rotation.

Supply voltage can range from 10V to 35V, while connection data for the MRS series is the same as for inductive three-wire sensors. Short-circuit protection is provided on the outputs. The MRS Series 300 and 350 come in sizes M12 x 1 and M18 x 1 in stainless steel V2A; Rechner Electronic also offers the Series 300 in size M12 x 1 in PA 6.6 material.

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